Volume force in continuous casting process

Hi
I am trying to run simulation of continuous casting process for L-shaped channel. I have been following instructions given for modelling of cylindrical rod (available on comsol website). As cylindrical rod simulation is done in 2-D, volume force is taken in 'r' and 'z' co-ordinates but I have to run it in 'x','y' and 'z'. I guess expression for 'z' will remain same here but no clue about 'x' and 'y'. Here what it is entered for cylindrical rod process (as per given in variables and parameters document uploaded on website)-

A_mush 6e4[kg/(m^3*s)] (Volume force damping constant)
alpha ht.alpha12 (Fraction of liquid phase)
epsilon 1e-3 (Volume force damping constant)
f (1-alpha)^2/(alpha^3+epsilon) ( Volume force prefactor)
Fr -f*A_mush*u (Volume force density, r-component)
Fz -f*A_mush*(w-v_cast) ( Volume force density, z-component)
v_cast = casting speed
u = velocity field
(no idea about w)
Kindly help me through this

Thank You
Shailesh